1. Field of the Invention
The present invention relates to methods of and systems for making thin films and multilayer coatings, and more specifically, it relates to a method for the production of axially symmetric, uniform or graded thickness thin film and multilayer coatings that avoid the use of apertures or masks to tailor the deposition profile.
2. Discussion of the Related Art
Thin film coatings are produced by various vapor-deposition methods (such as sputtering, chemical vapor deposition (CVD), electron beam evaporation, thermal evaporation, and ion beam plating) in which the substrate to be coated is passed through a vapor of the coating material and accumulates a thin film through condensation of the vapor. The ability to provide spatially graded thin film and multilayer coatings is essential for several advanced technologies. In particular, it is critical to developing optical fabrication techniques capable of fulfilling the stringent requirements of extreme ultraviolet lithography (EUVL) imaging systems, which require multiple aspheric optical elements figured to atomic-level accuracy. Multilayer coatings with the wrong thickness distribution would destroy the figure of the super-polished optics and would result in distorted lithographic patterns printed on the microelectronic circuits. In order to achieve the optimum imaging performance, graded period multilayer coatings must be deposited on these aspheric surfaces to within 0.1% accuracy, i.e., to within 0.01 nm for the typical coatings required for EUVL imaging optics.
Spatially graded thin film coatings are typically obtained with carefully shaped masks, or apertures, inserted between the sources and substrates. A different mask is required for each source-substrate combination. The masking operation requires iteration of the shape of the mask, and can be impractical for cases where perfect thickness distribution control is required at the location that coincides with the axis of rotation. At best, this is a tedious and inefficient process that is inappropriate for a robust manufacturing technique. The use of masks or baffles also lacks flexibility when several substrates must be coated in the same deposition run using several deposition sources. If differently customized masks are attached in close proximity to the substrates to intercept part of the incident deposition flux, custom coating gradients can be obtained for each substrate but one cannot independently control the flux emitted by each source. On the other hand, if differently customized masks are installed over the sources to shape the outgoing deposition flux, customized coating gradients can be obtained from each source but one cannot independently control the coating gradients deposited on several different substrates. Finally, since the source distribution flux can change over time as the source or target is consumed, the mask shape may require modification. This would require another mask fabrication and perhaps venting the chamber to atmosphere (undesirable) to install the new mask. As will be apparent from the description below, the present invention avoids use of baffles or masks, allows independent deposition of two different coating distributions on a substrate as the substrate sweeps sequentially across two sources during a single platter rotation, and also allows independent deposition of different coating distributions on several substrates coated in the same deposition run, each of which sweeps sequentially across a source.
U.S. Pat. No. 6,010,600, issued Jan. 4, 2000, to Vernon and Ceglio and U.S. patent application Ser. No. 09/454,673 by Walton, Montcalm and Folta disclose a technique for improving thickness distribution control during vapor deposition and circumvent the noted limitations of conventional masking methods. Instead of masking areas of the source flux, an optimal substrate sweep velocity recipe is determined and the substrate is swept through the deposition zone with a time-varying sweep velocity specified by such recipe. An aspect of the technique is a computer-implemented method for calculating the optimal substrate sweep velocity recipe for obtaining the desired thickness distribution profile. The technique disclosed in these references is specific to systems in which a substrate moves with time-varying sweep velocity across one or more stationary sources (or in which each source moves with time-varying sweep velocity relative to a stationary substrate), each of the sources emitting a fixed deposition flux. The inventors of the present invention, however, have recognized that it can be difficult to engineer and control such systems in which a load of multiple, heavy substrates (or vapor deposition sources) must be precisely accelerated and decelerated to various velocities within very short distances. The requirements for acceleration and deceleration of heavy substrates can significantly complicate the design of the substrates"" drive mechanism and increase the cost of the vapor deposition tool.
An important aspect of the present invention is another maskless approach for the production of laterally graded or uniform thin film coatings on arbitrarily shaped substrates, in which one modulates the power applied to each source (or otherwise modulates the flux distribution of each source) instead of modulating the sweep velocity of the substrate relative to each source (or of each source relative to the substrate). The present invention has all the advantages of the velocity modulation method described in the above-referenced parent application (over conventional masking methods), while eliminating the need for special mechanical drive requirements for modulating substrate (or source) sweep velocity.
Until the present invention, it had not been known how to achieve deposited coating thickness distribution control (graded or ungraded) of better than 0.1% across typical substrates (including curved substrates such as EUV optics as well as flat substrates), without the need for modulating substrate (or source) sweep velocity.
Prior to the present invention, it had been known to set the power applied to a vapor deposition source to achieve a desired deposition rate (a desired thickness per unit of time of a layer deposited on a substrate held fixed relative to the source). It had also been known to apply a level of power to a vapor deposition source which determines a desired deposition rate from a source, which in turn determines a nominal thickness of a layer (e.g., a thin film or one layer of a multilayer coating) deposited by the source onto a spinning substrate during a sweep (with fixed velocity) of the spinning substrate across the source. However, until the present invention it had not been proposed to modulate the power applied to a vapor deposition source (or otherwise to modulate the flux distribution of such a source) while sweeping a spinning or non-spinning substrate across the source, in order to deposit on the substrate a layer having a desired thickness distribution profile across the substrate.
It is expected that the invention will be useful in many applications, including precise deposition of laterally uniform or graded thin film coatings for EUV lithography, EUV optics, lithography masks, and optical coatings for general applications such as microscopy, astronomy, and spectroscopy, production of specifically graded coatings on curved optical elements, precise modification of the surface figure of optical elements for fabrication of aspheric optics, and production of extremely uniform films for semiconductor or magnetic recording devices.
In preferred embodiments, the invention is a method and system for depositing a thin film with highly accurate custom graded (or highly uniform) thickness over a substrate surface, by exposing the substrate to a region containing a vapor of the coating substance (referred to as a vapor deposition xe2x80x9csourcexe2x80x9d of coating material) in which the flux density is controlled. A thin film having a predetermined thickness profile is deposited on the substrate surface by sweeping the substrate across the source while controlling the source flux distribution in accordance with a selected source flux modulation recipe.
Typically, the source flux modulation recipe is a power modulation recipe specifying the power applied to the source as a function of time during the time interval in which the substrate sweeps across the source. In general, the source flux modulation recipe specifies how the flux distribution of the source varies over time (while the substrate sweeps across the source with constant velocity). This invention is applicable not only to flat substrates, but also to both concave and convex curved optics (i.e., optics having nonzero curvature).